FLOWERING PLANTS AND FERNS OF ARIZONA 687 



102. ASCLEPIADACEAE. Milkweed family 



Plants perennial, herbaceous or somewhat woody, mostly with 

 milky sap; leaves simple, entire-margined; flowers perfect, regular, 

 5-merous, usually in umbels, of highly specialized structure; stamens 

 and style coherent in a column, this adnate to the base of the corolla, 

 a crown (corona) of separate or united, often hoodlike appendages 

 usually present between the corolla and the column, and adnate to 

 one or the other or both; anthers commonly winged, often scarious- 

 tipped; pollen in pear-shaped masses (pollinia), usually 1 in each 

 anther cell, these attached in pairs to the summit of the column; 

 ovaries 2, united only by the common stigma; fruit a pair of follicles 

 (or 1 by abortion); seeds usually with a pappuslike crown of fine 

 bristles or hairs. 



The flowers of this family rival those of the Orehidaceae in com- 

 plexity of structure and in the manner in which the} 7 are adjusted for 

 cross-pollination by insects. The plants are probably somewhat 

 poisonous to livestock but are scarcely eaten except when other 

 forage is unavailable. 



Key to the genera 



1. Stems not twining, commonly erect; leaves never sagittate or deeply cordate 

 at base: anthers scarious-tipped, usually conspicuously so (2). 

 2. Corolla bearded within with stiff, white hairs; segments of the crown flat or 



slightly concave, not crested within 11. Pherotrichis. 



2. Corolla not bearded within, at most puberulent; segments of the crown 



strongly concave, at least at base (3). 



3. Hoods (segments of the crown) not appendaged within or, if so ('in Acerate s 



rusbyi) then the stems tall and wandlike, the leaves mostly alternate, 



narrowly linear and elongate, and the inflorescences lateral (4). 



4. Leaves all opposite, the blades oval or ovate: inflorescences terminal or 



subterminal; flowers dark red; hoods concave only near the base, 



strap-shaped above, much surpassing the column, with long narrow 



auricles at base ; anther wings widest at base. _ 2. Gomphocarpfs. 



4. Leaves mostly alternate or only approximately opposite, the blades 



lanceolate or narrower; inflorescences lateral; flowers greenish or 

 tinged with pale purple; hoods concave most of their length, not 

 surpassing the column, with short broad auricles at base: anther 



wings widest above the base 3. Acerates. 



3. Hoods crested within with a winglike or hornlike appendage; inflores- 

 cences often terminal as well as lateral (5). 



5. Corolla mostly reflexed in an thesis; anther wings widened down to the 



base, usually triangular; hoods involute or complicate, not arched 



at apex 4. Asclepias. 



5. Corolla rotate-spreading in an thesis; anther wings narrowed at base, 

 broadest near the middle: hoods arched and hollow at apex. 



5. ASCLEPIODORA. 



1. Stems twining; flowers lateral, solitary or clustered (6). 



6. Crown none, the campanulate corolla and the stamina! column unap- 

 pendaged; stems filiform; leaves narrowly linear, almost filiform; 

 flowers yellowish, less than 3 mm. long 1. Astbphanus. 



6. Crown present, sometimes reduced to subulate teeth; leaf blades cordate or 

 sagittate (7). 

 7. Anthers not or very inconspicuously scarious-tipped (8). 



8. Corolla greenish or brownish, rotate-campanulate or funnelform-cam- 

 panulate : segments of the crown each with a median winglike internal 

 crest ; stems puberulent or pilose with soft, more or less retrorse hairs. 



10. GONOLOBUS. 



8. Corolla whitish, funnelform-campanulate; segments of the crown not 

 wing-crested within: stems hirsute or pilose with stiff hairs. 



12. Lachnostoma. 



286744°— 42 44 



